1. Field of the Invention
The present invention relates generally to a non-linear distortion compensation circuit, a transmitter device to be employed in the same and a mobile communication unit. More particularly, the invention relates to a control system for a non-linear distortion compensation circuit for compensating non-linear distortion to be caused by non-linearity of an amplifier or a frequency converter.
2. Description of the Related Art
In general, it has been known a linearizer as a non-linear distortion compensation circuit for compensating non-linear distortion caused by non-linearity of an amplifier, a frequency converter or the like forming a transmitter to be used in a radio communication. As the linearizer, one taking output only non-linear signal from a transmission signal transmitted and compensating non-linear distortion by subtracting the taken out non-linear distortion signal, or one preliminarily multiplying a signal having distortion compensating characteristics to a transmission signal before radio transmission process, such as frequency conversion, amplification and so forth with taking a non-linear distortion to be caused in the transmitter into account, for compensating distortion.
On the other hand, it has been known that non-linear distortion caused by non-linearity of the amplifier, frequency converter and so forth forming the transmitter to be used in radio communication appears as a leak current in a frequency band of a channel on transmission and in a frequency band of an adjacent channel of the channel on transmission and the leak current increases according of increasing of transmission power. Particularly, a power leaking to adjacent channel is known as an adjacent-channel leak power. The adjacent-channel leak power caused by the transmitter may affect for other radio equipment communicating using the adjacent channel to cause degradation of reception characteristics.
Next, discussion will be given for influence of adjacent-channel leak power caused by radio equipment employing a spread spectrum communication system (CDMA system: Code Division Multiple Access system) performing multiplex communication by spreading spectra of communication signals as a communication system employed in a mobile communication, for other communication system.
In the CDMA system, spreading of spectra of communication signals is performed employing a spreading code, such as pseudo noise code (PN code: Pseudo random Noise code), and communication signals are identified by the spreading code. Therefore, the CDMA system is characterized by capability of simultaneous communication of a plurality of radio equipments and a plurality of channels at the same frequency. The CDMA system is also characterized by the fact that, upon demodulating a received signal, demodulation of the received signal cannot be accomplished unless multiplying the same spreading code as that used in spreading on transmission side at the same timing as that in the transmission side, and the received signal spread by different spreading code or the received signal spread at different timing, namely the signals used for communication by other radio equipments or the signals of other channels are fallen as noise within a reception band currently on reception.
Here, consideration is given for the case where a base station of mobile communication is present, where a plurality of mobile stations are present in distal position and proximal position of the base station and where communication is performed by CDMA system. When the mobile stations at distal position and the proximal position of the base station are communicating with the base station using the same frequency and the same transmission power, as viewed at a receiving end of the base station, the transmission power of the mobile station communicating at the proximal position of the base station is higher than that of the mobile station communicating at the distal position to submerge the transmission signal of the mobile station at the distal position in the transmission signal of the mobile station at the proximal position. This has been known as a far-near problem. In view of the characteristics of the CDMA system, even when the base station demodulates the signal transmitted from the distal mobile station, the transmission signal of the distal mobile station cannot be demodulated correctly since the signal of the proximal mobile station falls with the reception band as noise.
In the CDMA system, the far-near problem is solved by performing transmission power control at high precision and high accuracy relatively frequently. Namely, by performing transmission power control, a transmission power of the proximal mobile station is controlled to be lower power and a transmission power of the distal mobile station is controlled to be higher power. For transmission at higher power by the mobile station, the frequency converter and/or the amplifier forming the transmitter of the mobile station has to be operated at non-linear region thereof. As a result, non-linear distortion of the transmission signal increases at higher level of the transmission power. Namely, the adjacent-channel leak power transmitted from the transmitter is increased. Influence of the adjacent-channel leak power to other system will be discussed with reference to FIG. 6.
FIG. 6 illustrates a mobile communication system including a mobile station 101, a mobile station 102, a base station 103, a base station 104, a cell 105 and a cell 106. Here, consideration is given for the case where the base station 103 and the base station 104 perform communication using the CDMA system, and carriers of the base station 103 and the base station 104 are different. The cell 105 represents a service area of the base station 103 and is extended in the vicinity of the base station 104. On the other hand, the cell 106 is the service area of the base station 104.
When the mobile station 102 is in communication with the base station 104 at proximal position to the base station 104 and the mobile station 101 is in communication with the base station 103 with traveling from proximal position to the base station 103 to distal position thereof, the transmission power and the adjacent-channel leak power of the mobile station 101 increase according to traveling toward the distal position by transmission power control of the base station 103 to the mobile station 101. When the mobile station 101 approaches to the base station 104, the transmission power and the adjacent-channel leak power of the mobile station 101 arrive to the base station 104 at higher power.
If the communication frequency used by the base station 103 and the communication frequency used by the base station 104 are adjacent with each other, the transmission power transmitted from the mobile station 102 may submerge in the adjacent-channel leak power transmitted from the mobile station 101, In such case, a problem is encountered in that the base station 104 cannot receive the signal of the mobile station 102 correctly. This is because that the transmission power of the mobile station 102 is lower power for transmission power control of the base station 104 in the mobile station 102.
As a solution for this problem, a linearizer for compensating non-linear distortion of the transmitter is employed. As set forth above, as the linearizer, one taking output only non-linear signal from a transmission signal transmitted and compensating non-linear distortion by subtracting the taken out non-linear distortion signal, or one preliminarily multiplying a signal having distortion compensating characteristics to a transmission signal before radio transmission process, such as frequency conversion, amplification and so forth with taking a non-linear distortion to be caused in the transmitter into account, for compensating distortion. The former is not practical for increasing of current consumption due to increasing of circuit scale and for complexity of process. Therefore, the latter is primarily used as the non-linear distortion compensation circuit. Particularly, the later linearizer is called as pre-distortion type linearizer.
Prior art used in such pre-distortion type linearizer is shown in FIG. 7. It should be noted that the construction shown in FIG. 7 has been disclosed in Japanese Unexamined Patent Publication No. Heisei 10-23095. Referring to FIG. 7, a transmission signal is consisted with two-system of data, i.e. digital I data and Q data which are supplied to respective input terminals 21 and 22. These input data are supplied to a pre-distorter 25 via FIR filters 23 and 24 to obtain digital I data and Q data superimposed an inverted component of the non-linear distortion caused by a variable power amplifier 29.
To the pre-distorter 25, a transmission power control signal from a transmission power controller 34 is supplied in addition to I and Q data. The pre-distorter 25 derives the inverted component of the non-linear distortion by arithmetic process on the basis of the transmission power control signal and I and Q data. The inverted component thus derived is superimposed on the I and Q data. An output data of the pre-distorter 25 is converted into an analog signal by a D/A converter 26 and then is modulated by the modulator 27 with an output of an oscillator 28. Thus, I and Q signals (data) are modulated by orthogonal modulation. On the other hand, for CDMA system, spreading process by the spreading code is performed by the modulator 27.
Then, the transmission signal thus modulated by spreading is supplied to a variable power amplifier 29 to be amplified with an amplifier gain determined by a transmission power control signal from a transmission power controller 34. By this amplifier 29, non-linear distortion can be caused. An amplified output is mixed with an oscillation frequency from a frequency synthesizer 31 in a mixer 30 and then is transmitted as radio signal from an antenna 33 with amplification with a given gain by an amplifier 32.
It should be noted that the transmission power control signal output from the transmission power controller 34 is generated on the basis of a power control information bit input from a terminal 35 and a reception level information signal input from a terminal 36. Here, the power control information bit is a power control information bit data contained in the signal transmitted to own radio transmitter device from a counterpart (generally, the base station) to communicate. On the other hand, a reception level information signal is information of a reception level of a reception signal transmitted from the base station.
Thus, occurrence condition of non-linear distortion in the power amplifier 29 variable of amplifier gain can be accurately judged by the pre-distorter 25. Then, on the basis of occurrence condition of the non-linear distortion as judged, compensation component as inverted component of the non-linear distortion can be generated. With the compensation component, even when the amplifier gain is varied by the power amplifier 29, accurate distortion compensation corresponding to the amplifier gain can be performed.
Next, discussion will be given for a relationship between the conventional transmitter shown in FIG. 7 and a transmission power control effected for the mobile station (mobile communication equipment) by the base station. In case of CDMA system, the base station may discriminate the channel on reception or the mobile station in communication by the spreading code. Therefore, the base station may discriminate a power of a desired wave on reception and power of other reception signals, from the reception signal and thus can derive S/N ratio of the channel on reception. Transmission power control is performed depending upon the S/N ratio of the channel on reception. Namely, the base station derives a power of the desired wave and a power of interfering wave for deriving the S/N ratio of the channel on reception.
When the S/N ratio is less than or equal to the predetermined value, the base station transmits the control signal for increasing power of the transmission signal of the mobile station to the mobile station. On the other hand, when the S/N ratio is greater than the predetermined value, the base station transmits the information for reducing the transmission power so that the transmission power of the mobile station on communication will not interfere communication of other mobile station in communication. Calculation of S/N ratio executed by the base station is performed per time slot forming the transmission frame transmitted by the base station. Thus, the transmission control information is updated per time slot. A relationship between the transmission frame transmitted by the base station and the transmission power control information transmitted is shown in FIG. 8. Process of transmission power control to be executed will be discussed with reference to FIG. 8.
Referring to FIG. 8, one frame to be transmitted is consisted of n in number of time slots TS1 to TSn. One slot is consisted of the control information, transmission power control information and communication data. The mobile station on reception of the signal transmitted by the base station demodulats to extract the transmission power control information from the demodulated reception signal and performs transmission power control of the transmitter per time slot.
In this case, the compensation of the non-linear distortion performed on the basis of detected transmission power of the signal transmitted in the time slot TSi at a certain timing may compensate distortion of the transmission signal transmitted in the next time slot TS(i+1). However, since the transmission signal transmitted in the time slot TS(i+1) is differentiated the transmission power from the transmission power in the time slot TSi by the transmission power control, correct non-linear distortion compensation cannot be performed.
As set forth above, in the prior art, the non-linear distortion compensation to be performed on the basis of detected transmission power transmitted in the time slot TSi at a certain timing is effected for distortion compensation of the transmission signal of the transmission in the next time slot TS(i+1). However, since the transmission power transmitted in the time slot TS(i+1) is differentiated from the transmission power of the time slot TSi, correct non-linear distortion compensation cannot be performed.
On the other hand, it becomes important to perform the distortion compensation by the pre-distorter per bit (symbol) consisting the transmission data in each time slot of the transmission signal in order to make it accurate. However, in the construction shown in FIG. 7, compensation per bit (symbol) is not considered at all.
An object of the present invention is to provide a non-linear distortion compensation circuit, a transmission equipment employing the same and a mobile transmission equipment, in which a mobile station can accurately compensate distortion caused by non-linearity of a transmitter per bit (symbol) even while the base station is performing transmission power control for the transmitter of the mobile station.
According to the first aspect of the present invention, a non-linear distortion compensation circuit in a transmission equipment for controlling a transmission power depending upon an external transmission power control information upon transmission of a digital signal, comprises:
compensation component generating means for generating a compensation component for a non-linear distortion depending upon a transmission power per bit of the digital signal and the transmission power control information; and
compensating means for compensating the non-linear distortion of the transmission signal by the compensation component.
According to the second aspect of the present invention, a transmission equipment comprises:
a transmitter including component which causing non-linear distortion;
a non-linear distortion compensation circuit in a transmission equipment for controlling a transmission power depending upon an external transmission power control information upon transmission of a digital signal, including compensation component generating means for generating a compensation component for a non-linear distortion depending upon a transmission power per bit of the digital signal and the transmission power control information, and compensating means for compensating the non-linear distortion of the transmission signal by the compensation component.
According to the third aspect of the present invention, a mobile communication equipment comprises:
a receiver receiving a signal from a communication counterpart, the signal containing a transmission power control information;
a transmission equipment comprising a transmitter including component which causing non-linear distortion, a non-linear distortion compensation circuit in a transmission equipment for controlling a transmission power depending upon the transmission power control information upon transmission of a digital signal, including compensation component generating means for generating a compensation component for a non-linear distortion depending upon a transmission power per bit of the digital signal and the transmission power control information, and compensating means for compensating the non-linear distortion of the transmission signal by the compensation component.
The compensation component generating means may generate the compensation component per bit, and the compensation means may perform compensation of the non-linear distortion according to the compensation component per bit. The compensation component generating means may calculate the transmission power per bit on the basis of an instantaneous transmission power value and an average value of the transmission power, and generate the compensation component per bit according to the result of calculation and the transmission power control information. The compensation component generating means may comprise storage means for preliminarily storing a compensation data as the compensation component and address generating means for generating an address of the storage means on the basis of the transmission power per bit and the transmission power control information. The address generating means may be constructed to generate the address by addition of the transmission power per bit and the transmission power control information. The address generating means may derive the transmission power per bit is calculated by addition of an instantaneous transmission power and an average value of the transmission power. The transmission signal may be a time slot type, the external transmission power control information may be set for controlling the transmission power per transmission time slot, the address generating means may derive the transmission power per bit by addition of an average power value of a transmission time slot at a certain timing and an instantaneous power value per each bit of subsequent transmission time slot. The transmission power control information may be information for transmission power control for subsequent transmission time slot. The storage means may be a read-only memory.
The transmission power control signal may be respectively superimposed per time slot of the signal transmitted from the base station.
In the operation of the present invention, ROM address depending upon the transmission power per bit of the transmission signal and the transmission power control information is generated by the address generating portion generating the address of the compensation data storage ROM storing the non-linear distortion data to be used for performing non-linear distortion by the pre-distortion type linearizer. Particularly, the transmission power control information may reflect the time slot as control object.